A pulsed laser beam is often used by the enemy to target a tank, ship, air craft or strategic site and to guide the projectiles in attacking the above targets. Once the pulsed laser beam has been sent, the enemy's detection system will detect the reflected pulsed laser beam and analyze the delay to determine, for instance, the distance. Pulsed laser beams thus constitute threats in the battle field. In order to avoid the enemy attack, a jamming means or counter attack means must be exercised before or immediately after the attack. To achieve this effectively, it is required to identify the wavelength of the pulsed laser beam. For instance, if the wavelength of the threatening pulsed laser beam is known, then a pulsed laser tuned at the same wavelength can be switched on to emit pulses to confuse the enemy laser detection system. Alternatively, a counter attack means equipped with a laser beam seeking apparatus tuned at the enemy laser wavelength may be launched to destroy the enemy laser and detection system.
Spectral measurements of optical sources are generally made in three ways: [1] using an interferometer, [2] using a dispersing device and [3] using a filter device. Methods using the interferometer usually require delicate apparatus and precise motion of mirrors for the determination of wavelengths.
Most methods involving interferometers have several drawbacks which prevent their use in practical fields, for a instance, for the wavelength discrimination of single pulses or repetitive pulses. These drawbacks include: [1] the requirements of precise motion for the mirrors, [2] the incident angle of the beam must be known or adjusted with respect to the interferometer, [3] the equipment used is bulky. Methods involving the dispersing device or filter device for the laser spectral determination usually require an array of optical detectors, each connected to an amplifying and multiplexing circuit. For methods involving optical filters for separating the wavelengths, an optical filter with a very narrow pass band corresponding to the wavelength value to be detected is mounted in front of a detector. In order to achieve the wavelength resolution, the number of narrow band pass filters and the number of optical detectors must be large, making the wavelength determination unit too bulky and too slow to react to the incident laser threats. Different spectral analysis units using integrated detector arrays have been proposed. For instance, the U.S. Pat. No. 5,166,755 issued on Nov. 24, 1992 to Gat disclosed a spectrometer apparatus. The Gat apparatus consists of an array of monolithic photosensitive elements and a linear variable filter (LVF) where the number of photosensitive elements is at least the number of wavelengths to be discriminated. Therefore, the resolution of the Gat spectrometer is limited by the number of photosensitive elements current technology can produce and the relatively long total data acquisition time required. Since the number of photosensitive elements is large, the Gat spectrometer may not function when used in the detection of single laser pulses or repetitive pulses. Since the detector array is fabricated on a planar substrate, a finite gap exists between adjacent detector elements. When the position of the LVF for the maximum transmission of the incoming light falls within this gap, the accuracy of the wavelength determination may be reduced considerably.
A unit for the discrimination of a pulsed laser beams has been disclosed in U.S. Pat. No. 5,225,894 issued on Jul. 6, 1993 to Nicholson, Parker, Mathur and Hull. The Nicholson unit consists of broadband filters, optical fiber delay lines with different delay times and detectors. An incoming pulsed laser beam is divided into different portions and each is guided through a delay line. Signals from all of the delay lines are collected and compared with the signals obtained without the delay lines. From the delay time and the transmission properties of the optical filter, the wavelength of the incoming pulsed laser beam is deter mined. Although the number of the detectors is reduced as compared to the Gat apparatus, optical fiber delay lines with different lengths must be employed in the Nicholson unit. In order to obtain a significant time delay for an unequivocal delay time determination, the length of the optical fibers must be relatively large and a complicated time counting circuit must be included in the unit, making the Nicholson unit too bulky to be used in various field applications. In addition to the above, the enemy may launch repetitive pulses with very short intervals between the pulses. In such conditions, it may not be possible to discriminate the wavelength of the laser threats using the Nicholson unit, which relies on counting the time delay of well defined pulses for the wavelength determination. Another drawback of the Nicholson method is that it may not easily be extended to other wavelength regions, such as 3-5 .mu.m and 8-12 .mu.m, due to the various constraints for the delay lines.
In order to overcome the above drawbacks, a novel method using a detector pair has been developed for the determination of laser wavelength. The principle of the wavelength discriminating using the detector pair has been disclosed in a pending patent application Ser. No. 08/310,329, filed on Sep. 22, 1994 now U.S. Pat. No. 5,703,357. The active area of one of the detector elements increases with distance in one direction and the other decreases with the distance. A linear variable narrow band filter (LVF) is placed in front of the detector to define a light strip, the position of which is a function of the wavelength. By measuring the ratio of photo current of the two detector elements, the position of the light strip and thus the wavelength of the monochromatic light can be determined.
The above wavelength determination method still has some drawbacks. Firstly, the length of the detector elements will have to be approximately equal to the length of the LVF. The length of the LVF is determined by the range of wavelength to be covered and the wavelength resolution. For instance, a LVF for 0.6 to 1.1 .mu.m operation with a half height band width of 10 nm is about 15 mm. Secondly, only a small portion of the detector elements is illuminated by the incident light beam. For instance for the LVF of wavelength range 0.6 to 1.1 .mu.m and a half height band width of 10 nm, the ratio of the width of the illuminated detector and the total length is about 10 (nm)/[1100-600](nm)=1/50. Because of the above drawbacks, the detector elements are not operated in the best signal to noise ratio conditions. This is due to the presence of the leakage current and the parasitic capacitances associated with the regions not illuminated by the light.
In view of the above, there is a need to develop more advanced methods for the wavelength determination.